Use of oil-soluble surfactants as breaker enhancers for VES-gelled fluids

ABSTRACT

Fluids viscosified with viscoelastic surfactants (VESs) may have their viscosities reduced (gels broken) by the direct or indirect action of an internal breaker composition that contains at least one mineral oil, at least one polyalphaolefin oil, at least one saturated fatty acid and/or at least one unsaturated fatty acid. The internal breaker may initially be dispersed oil droplets in an internal, discontinuous phase of the fluid. In one non-limiting embodiment, the internal breaker, e.g. mineral oil, is added to the fluid after it has been substantially gelled. An oil-soluble surfactant is present to enhance or accelerate the reduction of viscosity of the gelled aqueous fluid.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/373,044 filed Mar. 10, 2006 (published Sep. 21, 2006 as U.S.Patent Application Publication No. 2006/0211776 A1), which in turnclaims the benefit of U.S. Provisional Application No. 60/662,336 filedMar. 16, 2005.

TECHNICAL FIELD

The present invention relates to gelled aqueous treatment fluids usedduring hydrocarbon recovery operations, and more particularly relates,in one non-limiting embodiment, to methods of “breaking” or reducing theviscosity of aqueous treatment fluids containing viscoelastic surfactantgelling agents used during hydrocarbon recovery operations throughinternal breakers and separate components that enhance the activity ofthe internal breakers.

TECHNICAL BACKGROUND

One of the primary applications for viscosified fluids is hydraulicfracturing. Hydraulic fracturing is a method of using pump rate andhydraulic pressure to fracture or crack a subterranean formation. Oncethe crack or cracks are made, high permeability proppant, relative tothe formation permeability, is pumped into the fracture to prop open thecrack. When the applied pump rates and pressures are reduced or removedfrom the formation, the crack or fracture cannot close or healcompletely because the high permeability proppant keeps the crack open.The propped crack or fracture provides a high permeability pathconnecting the producing wellbore to a larger formation area to enhancethe production of hydrocarbons.

The development of suitable fracturing fluids is a complex art becausethe fluids must simultaneously meet a number of conditions. For example,they must be stable at high temperatures and/or high pump rates andshear rates that can cause the fluids to degrade and prematurely settleout the proppant before the fracturing operation is complete. Variousfluids have been developed, but most commercially used fracturing fluidsare aqueous-based liquids that have either been gelled or foamed. Whenthe fluids are gelled, typically a polymeric gelling agent, such as asolvatable polysaccharide, for example guar and derivatized guarpolysaccharides, is used. The thickened or gelled fluid helps keep theproppants within the fluid. Gelling can be accomplished or improved bythe use of crosslinking agents or crosslinkers that promote crosslinkingof the polymers together, thereby increasing the viscosity of the fluid.One of the more common crosslinked polymeric fluids is boratecrosslinked guar.

The recovery of fracturing fluids may be accomplished by reducing theviscosity of the fluid to a low value so that it may flow naturally fromthe formation under the influence of formation fluids. Crosslinked gelsgenerally require viscosity breakers to be injected to reduce theviscosity or “break” the gel. Enzymes, oxidizers, and acids are knownpolymer viscosity breakers. Enzymes are effective within a pH range,typically a 2.0 to 10.0 range, with increasing activity as the pH islowered towards neutral from a pH of 10.0. Most conventional boratecrosslinked fracturing fluids and breakers are designed from a fixedhigh crosslinked fluid pH value at ambient temperature and/or reservoirtemperature. Optimizing the pH for a borate crosslinked gel is importantto achieve proper crosslink stability and controlled enzyme breakeractivity.

While polymers have been used in the past as gelling agents infracturing fluids to carry or suspend solid particles as noted, suchpolymers require separate breaker compositions to be injected to reducethe viscosity. Further, such polymers tend to leave a coating on theproppant and a filter cake of dehydrated polymer on the fracture faceeven after the gelled fluid is broken. The coating and/or the filtercake may interfere with the functioning of the proppant. Studies havealso shown that “fish-eyes” and/or “microgels” present in some polymergelled carrier fluids will plug pore throats, leading to impairedleakoff and causing formation damage.

Recently it has been discovered that aqueous drilling and treatingfluids may be gelled or have their viscosity increased by the use ofnon-polymeric viscoelastic surfactants (VES). These VES materials are inmany cases advantageous over the use of polymer gelling agents in thatthey are comprised of low molecular weight surfactants rather than highmolecular polymers. The VES materials may leave less gel residue withinthe pores of oil producing formations, leave no filter cake (dehydratedpolymer) on the formation face, leave a minimal amount of residualsurfactant coating the proppant, and inherently do not create microgelsor “fish-eyes”-type polymeric masses.

However, little progress has been made toward developing internalbreaker systems for the non-polymeric VES-based gelled fluids. To thispoint, VES gelled fluids have relied only on “external” or “reservoir”conditions for viscosity reduction (breaking) and VES fluid removal(clean-up) during hydrocarbon production. Additionally, over the pastdecade it has been found that reservoir brine dilution has only a minor,if any, breaking effect of VES gel within the reservoir.

Instead, only one reservoir condition is primarily relied on for VESfluid viscosity reduction (gel breaking or thinning), and that has beenthe rearranging, disturbing, and/or disbanding of the VES worm-likemicelle structure by contacting the hydrocarbons within the reservoir,more specifically contacting and mixing with crude oil and condensatehydrocarbons. SPE 30114 describes how reservoir hydrocarbons reduce theviscosity of VES-gelled fluids. SPE 31114 notes that when a VES-gelledfluid contacts crude or condensate reservoir hydrocarbons, theVES-gelled fluid will break, i.e. lose viscosity. SPE 60322 describeshow oil or gas reservoir hydrocarbons alter the worm-like micelles of aVES-gelled fluid into spherical micelle structures which results inwater-like fluid viscosity. SPE 82245 explains that contact of aVES-gelled fluid system with hydrocarbons causes the fluid to lose itsviscosity.

However, in many gas wells and in cases of excessive displacement ofcrude oil hydrocarbons from the reservoir pores during a VES geltreatment, results have shown many instances where VES fluid in portionsof the reservoir are not broken or are incompletely broken resulting inresidual formation damage (hydrocarbon production impairment). In suchcases post-treatment clean-up fluids composed of either aromatichydrocarbons, alcohols, surfactants, mutual solvents, and/or other VESbreaking additives have been pumped within the VES treated reservoir inorder to try and break the VES fluid for removal. However, placement ofclean-up fluids is problematic and normally only some sections of thereservoir interval are cleaned up, leaving the remaining sections withunbroken or poorly broken VES gelled fluid that impairs hydrocarbonproduction. Because of this phenomenon and other occasions wherereliance on external factors or mechanisms has failed to clean up theVES fluid from the reservoir during hydrocarbon production, or in caseswhere the external conditions are slow acting (instances where VESbreaking and clean-up takes a long time, such as several days up topossibly months) to break and then produce the VES treatment fluid fromthe reservoir, and where post-treatment clean-up fluids (i.e. use ofexternal VES breaking solutions) are inadequate in removing unbroken orpoorly broken VES fluid from all sections of the hydrocarbon bearingportion of the reservoir, there has been an increasing and importantindustry need for VES fluids to have internal breakers. Desirableinternal breakers that should be developed include breaker systems thatuse products that are incorporated within the VES-gelled fluid that areactivated by downhole temperature that will allow a controlled rate ofgel viscosity reduction over a rather short period of time of 1 to 8hours or so, similar to gel break times common for conventionalcrosslinked polymeric fluid systems.

A challenge has been that VES-gelled fluids are not comprised ofpolysaccharide polymers that are easily degraded by use of enzymes oroxidizers, but are comprised of low molecular weight surfactants thatassociate and form viscous rod- or worm-shaped micelle structures.Conventional enzymes and oxidizers have not been found to act anddegrade the surfactant molecules or the viscous micelle structures theyform. It is still desirable, however, to provide some mechanism thatrelies on and uses internal phase breaker products that will help assurecomplete viscosity break of VES-gelled fluids.

It would be desirable if a viscosity breaking system could be devised tobreak the viscosity of fracturing and other well completion fluidsgelled with and composed of viscoelastic surfactants, particularly breakthe viscosity completely and relatively quickly.

SUMMARY

There is provided, in one non-limiting form, a method for breaking theviscosity of aqueous fluids gelled with a viscoelastic surfactant (VES).The method includes adding to an aqueous fluid at least one VES in anamount that is effective to increase the viscosity thereof. At least oneinternal breaker is added to the aqueous fluid before or after the VESis added in an amount effective to reduce the viscosity of the gelledaqueous fluid. The internal breaker may be a mineral oil, a hydrogenatedpolyalphaolefin oil, a saturated fatty acid, an unsaturated fatty acidand combinations thereof. The mineral oil or hydrogenatedpolyalphaolefin oil may be advantageously added to the aqueous fluidafter it has been substantially gelled by the VES. The method furtherincludes adding to the fluid before, during or after adding the internalbreaker is added to the aqueous fluid, at least one oil-solublesurfactant (OSS) breaker enhancer in an amount effective to acceleratereduction of viscosity of the gelled aqueous fluid.

In another non-restrictive embodiment, there is provided an aqueousfluid that includes water and at least one viscoelastic surfactant (VES)in an amount effective to increase the viscosity of the aqueous fluid.The aqueous fluid also includes at least one internal breaker in anamount effective to reduce the viscosity of the gelled aqueous fluid.The internal breaker may again be a mineral oil, a hydrogenatedpolyalphaolefin oil, a saturated fatty acid, an unsaturated fatty acidsand combinations thereof. The aqueous fluid also includes an OSS breakerenhancer in an amount effective to accelerate reduction of viscosity ofthe gelled aqueous fluid.

The methods and compositions that will be described in further detailbelow allow the internal breaker to work at lower temperatures inrelatively high salinity brine mix waters. High salinity in the aqueousfluids has the effect of slowing the rate of breaking when internalbreakers are used in aqueous fluids gelled with a VES. The OSS breakerenhancer overcomes the rate-slowing effect that salinity has on theinternal breakers, particularly at lower temperatures. By using thesebreaker enhancers, internal breakers may now be used at ambienttemperatures (e.g. about 80° F. (about 27° C.)) to break VES-gelledfluids having high salinity. This permits the use of a lower amount ofthe internal breakers. This may have a number of benefits including, butnot necessarily limited to, lowering the overall cost to break aVES-gelled fluid by requiring less internal breaker when an OSS breakerenhancer is present, requiring less internal breaker to achieve acomplete viscosity breaker, and/or allowing complete VES-gelled fluidviscosity breaks to be achieved more quickly when using these internalbreakers, as compared to an identical method or composition where nobreaker enhancer is used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the viscosity breaking results as a functionof time using four different formulations of an Adriatic seawater brineincluding 4% KCl, 2.5% WG-3L VES, with varying amounts of a polyenoicacid internal breaker, without and with varying amounts of a transitionmetal ion source “catalyst” and with and without an OSS breakerenhancer;

FIG. 2 is a graph showing the viscosity breaking results as a functionof time using three different formulations of a brine having 9% KCl, 6%WG-3L VES, without and with 5.0 gptg FLC-40L fluid loss control agent,with and without internal breakers, without and with a transition metalion source “catalyst”, and without and with an OSS breaker enhancer; and

FIG. 3 is a graph showing the viscosity breaking results as a functionof time using three different formulations of a brine having 3% KCl, 4%WG-3L VES, without and with polyenoic acid internal breakers, withoutand with a transition metal ion source “catalyst”, and without and withvarious OSS breaker enhancers.

DETAILED DESCRIPTION

It has been discovered that oil-soluble surfactants (OSSs) may be usedas breaker enhancers for known internal breakers for VES-gelled aqueousfluids, such as the DiamondFRAQ™ technology available from Baker OilTools. The oil-soluble surfactants may allow lower internal breakerconcentrations to be used to achieve quick and complete VES-gelled fluidbreaks (viscosity reductions). This family of oil-soluble surfactantshas been discovered to activate polyenoic acids and mineral oil internalbreakers so that they may now be used in high salinity brines down toambient temperatures. This expands and enhances these internal breakingtechnologies. The oil-soluble surfactants are relatively inexpensive,may be used in relatively low concentrations, and may lower the cost ofusing the internal breakers.

More specifically, the oil-soluble surfactant breaker enhancers mayallow the internal breakers to work at lower temperature in highsalinity brine mix waters, in non-limiting examples from about 1% toabout 24% by weight (bw) KCl; about 1% to about 44% bw NaBr, about 1% toabout 37% bw CaCl₂ brine; about 1% to about 63% bw CaBr₂ brine; etc.These oil-soluble surfactant breaker enhancers may overcome therate-slowing effect salinity has on internal breakers at lowertemperatures, and thus mineral oil-type internal breakers andunsaturated fatty acid-type breakers may now be used at ambienttemperatures, such as about 80° F. (about 27° C.), to break VES fluidsthat have high salinity. By using the oil-soluble surfactant breakerenhancers, lower amounts of the polyenoic acid and/or mineral oilbreakers may be used. This will give advantages that include, but arenot necessarily limited to, lowering the overall cost to break aVES-gelled aqueous fluid by requiring less internal breaker when an OSSis present, lowering the amount of internal breaker to achieve completeVES viscosity break, allowing a complete VES fluid viscosity break to beachieved more quickly than when using polyenoic or mineral oil breakersalone and combinations of these.

The viscoelastic surfactants are believed to impart viscosity to anaqueous fluid by the molecules organizing themselves into micelles.Spherical micelles do not give increased viscosity, however, when themicelles have an elongated configuration, for instance are “rod-shaped”or “worm-shaped”, they become entangled with one another therebyincreasing the viscosity of the fluid.

In one non-limiting embodiment gel-breaking products, such as theinternal breakers herein, work by rearrangement of the VES micelle fromrod-shaped or worm-shaped elongated structures to spherical structures:that is, the collapse or rearrangement of the viscous elongated micellestructures to nonviscous, more spherical micelle structures.Disaggregating may be understood in one non-limiting embodiment when themicelles are not closely associated physically, that is no longeraggregated or physically interacted together thereby resulting inreduced fluid viscosity, as contrasted with rearrangement which may beunderstood as a different physical and chemical arrangement oraggregation of the multi-surfactant micelles that has reduced viscosity.However, the inventors do not necessarily want to be limited to anyparticular mechanism or explanation.

Elongated VES structures may sometimes be referred to as “living”because there is a continuous exchange of VES-type surfactants leavingthe VES micelle structures to the aqueous solution and other surfactantsleaving the aqueous solution and entering or re-entering the VES micellestructures. It is suspected, in one non-restrictive explanation, thatthe oil-soluble surfactants over time and under specific conditionsbecome dispersed in the VES elongated micelles and thereby allow a smallopening to occur in the VES micelles that allows the internal breakersto enter and/or further destabilize the VES micellar structure, somewhatanalogous to a chemical “pinhole” or localized position that opens up orfacilitates disruption of the micelles by the internal breakers.However, the inventors do not wish to be limited to any particularexplanation or mechanism. With mineral oil internal breakers,particularly the higher molecular weight mineral oils, it appears thatthe oil-soluble surfactants lower the free energy required to penetratethe VES micelle head groups, and allows or permits, or appears totransport or carry the higher molecular weight mineral oil moleculesinto the VES micelle. Thus, the ability of the mineral oil internalbreaker to penetrate and break the VES structures and to break thesestructures more quickly is enhanced. In the case of the polyenoic acidinternal breakers, it may be that the oil-soluble surfactants hold orretain the auto-oxidized fatty acid compounds (i.e. thehydrophobic-saturated hydrocarbon autooxidation byproducts) to disturbthe VES micelle head and/or tail group attractions, thus enhancing thepolyenoic auto-oxidation byproduct compound's ability or activity tomore readily break the VES micelle structure. Again, it seems to be thatthe oil-soluble surfactants distribute and create “pinhole” or localizedweakened VES head group arrangements or configurations in the VESstructure that allows the internal breaking agents to associate andfurther weaken the “pinhole” locally. In any event, the overall resultis that the internal breakers that normally do not work or very slowlywork are able to work and/or work more quickly in degrading the VESfluid viscosity when the oil-soluble surfactants are present in thefluid. That is, the breaker enhancers overcome the negative influencesof relatively high salinity and relatively low fluid temperature,particularly for high molecular weight type internal breakers; theyallow the internal breakers to work more effectively and/or more quicklyat lower temperatures, lower concentrations and/or higher mix watersalinity. The result is that internal breakers may be used in situationsor environments when they normally are ineffective or are effective onlyat very high concentrations. This discovery expands the domain and useof internal breakers and reduces the cost of using them.

The internal breaker components herein may be added safely and easily tothe gel after batch mixing of a VES-gel treatment, or added on-the-flyafter continuous mixing of a VES-gel treatment using a liquid additivemetering system in one non-limiting embodiment, or the components may beused separately, if needed, as an external breaker solution to removeVES gelled fluids already placed downhole. The mineral oils (beinginherently hydrophobic) and/or the mono- and/or polyenoic acids are notsolubilized in the brine, but rather interact with the VES surfactantand/or remain as oil droplets to be dispersed and form an emulsion (oilin water type emulsion) and thus there is an oil-stabilized emulsiondispersed in the “internal phase” as a “discontinuous phase” of thebrine medium/VES fluid which is the “outer phase” or “continuous phase”.It appears in some cases the mineral oils, e.g., or unsaturated fattyacids (UFAs) are evenly dispersed and are incorporated within theviscous rod- or worm-like shape micelles, however, in other cases,particularly for mineral oils and other saturated hydrocarbons, the oilbreaker component can remain as droplets outside of the VES micelles, oras a combination of both micellular locations to various degrees.Rheometer tests have shown, that the incorporation of the UFAs into(within or a part of) the VES micelles does not disturb the viscosityyield of the VES micelles at the levels or amounts of UFAs needed toobtain a complete VES gel viscosity break. This is remarkably uniquesince oils are considered to readily break the viscosity of VES fluids.The UFAs in particular, have high compatibility with VES fluids untilthey undergo natural or induced autooxidation, whereby the autooxidationbyproducts have been found to readily disturb VES micelles structuresand fluid viscosity.

It is surprising and unexpected that mineral oils may serve as internalbreakers. This is surprising because the literature teaches that“contact” of a VES-gelled fluid with hydrocarbons, such as those of theformation in a non-limiting example, essentially instantaneously reducesthe viscosity of the gel or “breaks” the fluid. By “essentiallyinstantaneously” is meant less than one-half hour. The rate of viscositybreak for a given reservoir temperature by the methods described hereinis controlled by type and amount of salts within the mix water (i.e.seawater, KCl, NaBr, CaCl₂, CaBr₂, NH₄Cl and the like), presence of aVES gel stabilizer (i.e. MgO, ZnO and the like), presence of aco-surfactant (i.e. sodium dodecyl sulfate, sodium dodecyl benzenesulfonate, potassium laurate, potassium oleate, sodium lauryl phosphate,and the like), VES type (i.e. amine oxide, quaternary ammonium salt, andthe like), VES loading, the amount of mineral oil used, the distillationrange of the mineral oil, its kinematic viscosity, the presence ofcomponents such as aromatic hydrocarbons, and the like. Additionally, asdisclosed the rate of viscosity break can be controlled by type andamount of breaker enhancer used.

It is important in most non-limiting embodiments herein to add the lowermolecular weight or low viscosity mineral oil products after the aqueousfluid is substantially gelled. Addition of the lower molecular weightmineral oil prior to substantial gelling tends to prevent the gelling orviscosity increase to occur. By “substantially gelled” is meant that atleast 90% of the viscosity increase has been achieved before theinternal breaker (e.g. mineral oil) is added. Of course, it isacceptable to add the lower molecular weight mineral oil after the gelhas completely formed. However, in another non-limiting embodiment, whenusing the higher molecular weight or higher viscosity mineral oils theorder of addition is not important, that is, these type of mineral oilsmay be added prior to, during, or after the VES product is added to theaqueous fluid and gelled.

Mineral oil (also known as liquid petrolatum) is a by-product in thedistillation of petroleum to produce gasoline. It is a chemically inerttransparent colorless oil composed mainly of linear, branched, andcyclic alkanes (paraffins) of various molecular weights, related towhite petrolatum. Mineral oil is produced in very large quantities, andis thus relatively inexpensive. Mineral oil products are typicallyhighly refined, through distillation, hydrogenation, hydrotreating, andother refining processes, to have improved properties, and the type andamount of refining varies from product to product. Highly refinedmineral oil is commonly used as a lubricant and a laxative, and withadded fragrance is marketed as “baby oil” in the U.S. Most mineral oilproducts are very inert and non-toxic, and are commonly used as babyoils and within face, body and hand lotions in the cosmetics industry.Other names for mineral oil include, but are not necessarily limited to,paraffin oil, paraffinic oil, lubricating oil, white mineral oil, andwhite oil.

In one non-limiting embodiment the mineral oil may be at least 99 wt %paraffinic. Because of the relatively low content of aromatic compounds,mineral oil has a better environmental profile than other oils. Ingeneral, the more refined and less aromatic the mineral oil, the better.In another non-restrictive version, the mineral oil may have adistillation temperature range from about 160 to about 550° C.,alternatively have a lower limit of about 200° C. and independently anupper limit of about 480° C.; and a kinematic viscosity at 40° C. fromabout 1 to about 250 cSt, alternatively a lower limit of about 1.2independently to an upper limit of about 125 cSt. Specific examples ofsuitable mineral oils include, but are not necessarily limited to,BENOL®, CARNATION®, KAYDOL®, SEMTOL®, HYDROBRITE® and the like mineraloils available from Crompton Corporation, ESCAID®, EXXSOL® ISOPAR® andthe like mineral oils available from ExxonMobil Chemical, and similarproducts from other mineral oil manufacturers. A few non-limitingexamples are specified in Table I. The ESCAID 110® and CONOCO LVT-200®mineral oils have been well known components of oil-based drilling mudsand the oil industry has considerable experience with these products,thus making them an attractive choice. The white mineral oils fromCrompton Corporation with their high purity and high volume use withinother industries are also an attractive choice.

TABLE I PROPERTIES OF VARIOUS MINERAL OILS ESCAID EXXSOL HYDROBRITEHYDROBRITE 110 D110 ISOPAR V BENOL 200 1000 Properties Specific Gravity0.790-0.810 0.780-0.830 0.810-0.830 0.839-0.855 0.845-0.885 0.860-0.885Viscosity @ 1.3-1.9 — — 18.0-20.0 39.5-46.0 180.0-240.0 40° C. FlashPoint (° C.) 77.0 105 118 186 — 288 Pour Point (° C.) — — — −21.0 −9.0−6.0 Distillation Range IBP (° C.) 200 237 263 — — — Max DP (° C.) 248277 329 — — — GC Distillation — — — — >380 >407 5% (° C.) Molecular Wt.— — — — — >480 Aromatic <0.5% <1.0% <0.5% — — — Content Note: ESCAID,EXXSOL and ISOPAR are trademarks of ExxonMobil Corporation. BENOL andHYDROBRITE are trademarks of Crompton Corporation.

It has been discovered in breaking VES-gelled fluids prepared inmonovalent brines (such as 3% KCl brine) that at temperatures belowabout 180° F. (82° C.) ESCAID® 110 mineral oil works well in breakingVES-gelled fluids, and that at or above about 140° F. (60° C.)HYDROBRITE® 200 mineral oil works well. The use of mineral oils hereinis safe, simple and economical. In some cases for reservoir temperaturesbetween about 120° to about 240° F. (about 49° to about 116° C.) aselect ratio of two or more mineral oil products, such as 50 wt %ESCAID® 110 mineral oil to 50 wt % HYDROBRITE® 200 mineral oil may beused to achieve controlled, fast and complete break of a VES-gelledfluid. The use of the breaker enhancers herein expands the ranges oftemperatures and brine salt concentrations for which these mineral oilsare useful.

It has also been discovered that the type and amount of salt within themix water used to prepare the VES fluid (such as 3 wt % KCl, 21 wt %CaCl₂, use of natural seawater, and so on) and/or the presence of a VESgel stabilizer (such as VES-STA 1 available from Baker Oil Tools) mayaffect the activity of a mineral oil in breaking a VES fluid at a giventemperature. For example, ESCAID® 110 mineral oil at 5.0 gptg willreadily break the 3 wt % KCL based VES fluid at 100° F. (38° C.) over a5 hour period, and ESCAID® 110 mineral oil may still have utility as abreaker for a 10.0 ppg CaCl₂ (21 wt % CaCl₂) based VES fluid at 250° F.(121° C.).

Other refinery distillates may potentially be used in addition to oralternatively to the mineral oils described herein, as may behydrocarbon condensation products. Additionally, synthetic mineral oils,such as hydrogenated polyalphaolefins, and other synthetically derivedsaturated hydrocarbons may be of utility to practice the methods herein.More information about the use of mineral oils, hydrogenatedpolyalphaolefin oils, and saturated fatty acids as internal breakers maybe found in U.S. Patent Application Publication 2007/0056737 A1published Mar. 15, 2007, incorporated by reference herein in itsentirety. In another non-limiting embodiment, natural unsaturatedhydrocarbons such as terpenes (e.g. pinene, d-limonene, etc.), saturatedfatty acids (e.g. lauric acid, palmitic acid, stearic acid, etc. fromplant, fish and/or animal origins) and the like may possibly be usedtogether with or alternatively to the mineral oils herein.

With respect to the unsaturated fatty acid internal breakers such asmonoenoic acids and polyenoic acids, as internal breakers, in onenon-limiting embodiment these may be specific oils that contain arelatively high amount of either monoenoic or polyenoic acids or both.There are many books and other literature sources that list the multipletypes and amounts of fatty acids compositions of oils and fats availablefrom plant, fish, animal, and the like. A polyenoic acid is definedherein as any of various fatty acids having more than one double bond(allyl group) in the carbon chain, e.g. in linoleic acid.Correspondingly, a monoenoic acid is a fatty acid having only one doublebond (allyl group). The terms unsaturated fatty acid (UFA) orunsaturated fatty acids (UFAs) are defined herein as oils or fatscontaining one or the other or both monoenoic and polyenoic fatty acids.Other suitable polyenoic acids include, but are not necessarily limitedto omega-3 fatty acids, and omega-6 fatty acids, stearidonic acid,eleostearic acid, eicosadienoic acid, eicosatrienoic acid, arachidonicacid or eicosatetraenoic acid (ETA), eicosapentaenoic acid (EPA),docosapentaenoic acid, docosahexaenoic acid (DHA), cis-linoleic acid,cis-linolenic acid, gamma-linolenic acid, conjugated polyenes, andmixtures thereof. Other suitable monoenoic acids include, but are notnecessarily limited to obtusilic acid, caproleic acid, lauroleic acid,linderic acid, myristoleic acid, physeteric acid, tsuzuic acid,palmitoleic acid, petroselinic acid, oleic acid, vaccenic acid, gadoleicacid, gondoic acid, cetoleic acid, nervonic acid, erucic acid, elaidicacid, t-vaccenic acid, and mixtures thereof.

Oils relatively high in monoenoic and polyenoic acids include, but arenot necessarily limited to, flax (linseed) oil, soybean oil, olive oil,canola (rapeseed) oil, chia seed oil, corn oil, cottonseed oil, eveningprimrose oil, grape seed oil, pumpkin seed oil, safflower oil, sunfloweroil, walnut oil, peanut oil, various fish oils, mammal oils, and animaloils or fats and the like.

Any of these oils or fats may be partially hydrogenated, or may containoriginal or additional preservatives, such as tocopherols, and the like.Additionally any one or more of these oils may be “aged” before use toadjust the product's auto-oxidation activity, along with any one or morereagent or technical grade fatty acids. Allowing a specific fatty acidor UFA oil to “age” allows auto-oxidation to initiate and progressdependant on the amount of time, environmental conditions (temperature,exposure to atmosphere, etc.), presence of other compounds (tocopherols,metal ions, etc), and the like.

It appears that the more double-bonded carbons on the fatty acid carbonchain the more active that fatty acid will be in auto-oxidation, thatis, these materials auto-oxidize easier and more quickly. This seems tobe a general rule, although other components in the oil may alter thisrule. Table II lists the relative rates of oxidation of common fattyacids, from the “Autooxidation” section within “Chemical Reactions ofOil, Fat, and Based Products”, Department of Engineering, InstitutoSuperior T'echnico, Lisbon, Portugal, October 1997.

TABLE II RELATIVE OXIDATION RATES OF SOME COMMON FATTY ACIDS Totalamount of Number of double Relative rate of Fatty acid carbon atomscarbon bonds oxidation Stearic 18 0 1 Oleic 18 1 100 Linoleic 18 2 1200Linolenic 18 3 2500

Unsaturated fatty acids have been found to break down by“auto-oxidation” into a gamut of VES-breaking products or compositions.Each oil with various monoenoic and polyenoic acids uniquely shows thebreakdown of the VES surfactant micelle structure by the presence ofthese auto-oxidation generated byproducts. Auto-oxidation is also knownas autooxidation and lipid peroxidation which includes the oxidation ofunsaturated fatty acids. Auto-oxidation in this context may also includea chain reaction—multiple steps and chemical species occur in theoxidative breakdown. Various hydroperoxides may be formed in theseauto-oxidations, and end products typically include, but are notnecessarily limited to, carbonyl compounds (various aldehydes andketones), alcohols, acids, and “hydrocarbons” of various types, e.g.alkanes, saturated fatty acids and the like, and mixtures thereof. Avariety of technical books and papers list many of the numerous productsgenerated by auto-oxidation (autooxidation) of unsaturated fatty acids.

Fatty acids may also decompose in a water medium and alkaline conditionby hydrolysis.

It may be possible that other olefins (e.g. allyl group compounds) maybe investigated and employed in the same manner that unsaturated fattyacids have been found to work toward breaking VES-gelled fluids. It alsomay be possible that mechanisms other than oxidation or hydrolysis maybe functioning in generating VES breaking compounds from olefins andolefin derivatives, although the inventors do not want the methods andcompositions herein to be limited by any supposed theory.

More information about the use of mono- and polyenoic acids (UFAs) asinternal breakers may be found in U.S. Patent Application Publication2006/0211776 A1 published Sep. 21, 2006, incorporated by referenceherein in its entirety.

The breaking or viscosity reduction may be triggered or initiated byheat for both types of internal breakers. The mineral oils and relatedinternal breakers will slowly, upon heating, break or reduce theviscosity of the VES gel with the addition of or in the absence of anyother viscosity reducing agent. The use of the oil-soluble enhancerswill further influence the triggering and rate of viscosity reduction byboth types of internal breakers. In general, the amount of mineral oilneeded to break a VES-gelled fluid appears to be temperature dependent,with less needed as the fluid temperature increases, unless one of thebreaker enhancers described herein is included. The kinematic viscosity,molecular weight distribution, and amount of impurities (such asaromatics, olefins, and the like) also appear to influence the rate inwhich a mineral oil will break a VES-gelled fluid at a giventemperature. Once a fluid is completely broken a degree of viscosityreheal may occur but in most cases no rehealing is expected because, asnoted, it is difficult or impossible to create a gelled fluid in thepresence of mineral oil in the first place. An effective amount ofmineral oil ranges from about 0.1 to about 15 gptg based on the totalfluid, in another non-limiting embodiment from a lower limit of about0.5. Independently the upper limit of the range may be about 10 gptgbased on the total fluid. The necessary proportions or amounts areexpected to be lower in the presence of breaker enhancers. (It will beappreciated that units of gallon per thousand gallons (gptg) are readilyconverted to SI units of the same value as, e.g. liters per thousandliters.)

The mono- and polyenoic acids will slowly to fairly rapidly, uponheating or subjecting the acids to a temperature, auto-oxidize into theVES gel breaking compounds with the addition of or in the absence of anyother agent. The amount of altered or oxidized unsaturated fatty acidneeded to break a VES-gelled fluid appears to be VES concentration andtemperature dependent, with typically more needed as the VESconcentration increases and less needed as fluid temperature increases.Of course, as previously discussed, the presence of breaker enhancerswill facilitate the reduction of the VES-caused viscosity and lower thetemperatures necessary for breaking to occur. Once a fluid is completelybroken a degree of viscosity reheal may occur but in most cases noreheal in viscosity will occur and no phase separation of the VES occursupon fluid cool down, that is when the test fluid is left at testtemperature for a sufficient amount of time for complete tonear-complete auto-oxidation of the monoenoic and/or polyenoic acids tooccur.

The viscosity reduction or breaking action of the previously discussedinternal breakers may be enhanced using the oil-soluble surfactants ofthe present methods and compositions. Suitable oil-soluble surfactantbreaker enhancers may include, but are not necessarily limited to,sorbitan fatty acid esters, saponified hard oils, saponifiedhydrogenated fatty acid oils, long chain alcohols, long chain fatty acidalcohols, long chain fatty amines, long chain sulfates, long chainsulfonates, phosopholipids, and lignins. Specific suitable OSSs include,but are not limited to, sorbitan fatty acid esters available fromUniqema as SPAN® 20 sorbitan monolaurate, SPAN® 40 sorbitanmonopalmitate, SPAN® 61 sorbitan monostearate, SPAN® 65 sorbitantristearate, SPAN® 80 sorbitan monooleate, SPAN® 85 sorbitan trioleate,TWEEN® 20 sorbitan monolaurate (with 20 POE (polyoxyethylene) units),TWEEN® 21 sorbitan monolaurate (POE (4)), TWEEN® 40 sorbitanmonopalmitate (POE (20)), TWEEN® 60 sorbitan monostearate (POE (20)),TWEEN® 61 sorbitan monostearate (POE (4)), TWEEN® 65 sorbitantristearate (POE (20)), TWEEN® 80 sorbitan monooleate (POE (20)), TWEEN®81 sorbitan monooleate (POE (4)), TWEEN® 85 sorbitan trioleate (POE(20)).

Other suitable OSSs include, but are not necessarily limited to,saponified hard oils, which include reactions of saponifying agents likesodium, potassium, calcium, etc. hydroxides with “hard oils” like mahua,tallow, castor, Neem, and the like; saponified hydrogenated fatty acidoils (with the above saponifying agents); partially and/or fullyhydrogenated oils like coconut, cottonseed, soybean, and the like; longchain alcohols including long chain linear and branched alcohols thathave been ethoxylated and/or propoxylated; long chain fatty amines,including dimeric forms (where two long chain fatty amine surfactantsare combined at the head groups); long chain sulfates and sulphonates;and phospholipids, lignins, and mixtures of these.

The proportions of the breaker enhancers useful in the methods andcompositions herein range from about 0.001 to about 10% by weight, basedon the total VES-gelled fluid composition, and alternatively in onenon-limiting embodiment may range from a lower limit of about 0.01,independently to an upper limit of about 2.0% by weight. The amount ofOSS needed to enhance the polyenoic breakers, mineral oil breakers andthe like internal phase VES breakers will depend on a number of diverse,but interrelated factors including, but not necessarily limited to, typeof VES, VES loading, mix water salinity, fluid temperature, the type ofOSS, etc. The matrix that the oil-soluble surfactant originates fromwill also affect the amount to be used, i.e. if the OSS is alreadywithin high molecular weight mineral oil before OSS addition to VESfluid, this may slow diffusion of the OSS into VES micelles, etc. TheOSS may be added before, during or after addition of the internalbreaker to the aqueous fluid gelled with the VES, and as noted, may becombined with the internal breaker prior to addition. In onenon-limiting embodiment the oil-soluble surfactant may be advantageouslyadded to high molecular weigh mineral oil, such as ConocoPhillips PurePerformance Base Oil 225N or 600N, at approximately 10 to 20% by volumeOSS to 225N or 600N mineral oil.

Controlled viscosity reduction rates using the internal breakers may beachieved at a temperature of from about 70° F. to about 300° F. (about21 to about 149° C.), and alternatively at a temperature of from about100° F. independently to an upper end of the range of about 280° F.(about 38 to about 138° C.). The temperature range for a given internalbreaker may be shifted lower substantially with the type and amount ofthe breaker enhancers used herein. It has also been discovered thatVES-gelled aqueous fluids containing the small amounts of mineral oilsdescribed herein are relatively shear stable and can tolerate some shearbefore viscosity reduction occurs. In one non-limiting embodiment, thefluid designer would craft the fluid system in such a way that the VESgel would break at or near the expected formation temperature afterfracturing was accomplished.

Fluid design may be based primarily on formation temperature, i.e. thetemperature the fluid will be heated to naturally in the formation oncethe treatment is over. Further, fluid design may be based on theexpected cool down of the fluid during a treatment. In many cases thefracturing fluid may only experience actual reservoir temperature for 5%to 25% of the job time, and close to 50% of the fluid is never exposedto the original reservoir temperature because of the cool down of thereservoir by the initial fracturing fluid placed into the reservoir. Itis because a portion of the fracturing fluid will not see or be exposedto the original reservoir temperature that a cooler temperature isselected that will represent what the fluid will probably see orcontact, and thus laboratory break tests are sometimes run at a coolertemperature. There would generally be no additional temperature the VESfluid would see other than original reservoir temperature.

The use of the disclosed internal breaker/breaker enhancer system isideal for controlling viscosity reduction of VES-based fracturingfluids. The breaking system may also be used for breaking gravel packfluids, acidizing or near-wellbore clean-up diverter fluids, and losscirculation pill fluids composed of VES. The breaker system mayadditionally work for foamed fluid applications (hydraulic fracturing,acidizing, and the like), where N₂ or CO₂ gas is used for the gas phase.The VES breaking methods and compositions herein are a significantimprovement in that they give breaking rates for VES-based fluids thatthe industry is accustomed to with conventional polymer based fracturingfluids, such as borate crosslinked guar. Potentially more importantly,in another non-limiting example, the use of the internal breaker/breakerenhancer systems should help assure and improve hydrocarbon productioncompared to prior art that uses only external mechanisms to break theVES fluid for effective and complete VES fluid clean-up after atreatment.

In one non-limiting embodiment of the invention, the compositions hereinwill directly degrade the gel created by a VES in an aqueous fluid, andalternatively will reduce the viscosity of the gelled aqueous fluideither directly, or by disaggregation or rearrangement of the VESmicellar structure (e.g. collapsing or disturbing the structure).However, the inventors do necessarily not want to be limited to anyparticular mechanism.

In another non-limiting embodiment, the composition may be modified toslow down or to increase the auto-oxidation of the unsaturated fattyacids. Addition of compounds that influence the rate of auto-oxidationis an important option for the methods and fluids herein, in particularfor the lower temperatures to increase the auto-oxidation rate and athigher temperatures to slow down the auto-oxidation rate. Rate controlcompounds that may be used for slowing down rate of monoenoic andpolyenoic acids may be antioxidants such as, but not limited totocopherol (vitamin E), ascorbic acid (vitamin C), butylatedhydroxytoluene (BHT) and other like preservatives, chelants (such ascitric acid, phosphates, and EDTA), amino acids, proteins, sugaralcohols (e.g. mannitol, xylitol, lactitol, and sorbitol), salts (suchas NaCl, MgCl₂, CaCl₂, NaBr and CaBr₂), and the like. Rate controlcompounds that may increase the rate of auto-oxidation may be oxidantsor pro-oxidants such as, but not limited to persulfate, percarbonate,perbromate, iron, copper, manganese and other transition metals, and thelike. It should be noted that there are numerous compounds that may beof utility for regulating the rate of auto-oxidation. The proportion ofrate control compounds that may be advantageously used may range from alower limit of about 0.00001% by weight to an upper limit of about 62%by weight, based on the total weight of fluid, and alternatively from alower limit of 0.0001% by weight and/or to an upper limit of about 45%by weight. It may be noted that rate controllers used toward the lowerlimit may be items such as metal ions and rate controllers employedtoward the upper limit may be items such as monovalent and/or divalentsalts. Chelation of the metal ions tends to slow the rate ofauto-oxidation as compared with non-chelated forms of the same metalions. In one non-limiting understanding, the use of metal ions (whetheror not chelated) may be understood as “catalyzing” the auto-oxidation ofthe UFA.

It is sometimes difficult to specify with accuracy in advance the amountof the various internal breaking components that should be added to aparticular aqueous fluid gelled with viscoelastic surfactants tosufficiently or fully break the gel, in general. For instance, a numberof factors affect this proportion, including but not necessarily limitedto, the particular VES used to gel the fluid; the particular internalbreaker used; the particular oil used as a carrier in the case of theunsaturated fatty acids; whether or not a rate-controlling agent is usedand what kind; the temperature of the fluid; the downhole pressure ofthe fluid, the starting pH of the fluid; and the complex interaction ofthese various factors.

Nevertheless, in order to give an approximate feel for the proportionsof the various breaking components to be used in the method of theinvention, approximate ranges will be provided. In an alternative,non-limiting embodiment the amount of mineral oil, hydrogenatedpolyalphaolefin oil and saturated fatty acid that may be effective inthe methods and compositions herein may range from about 5 to about18,000 ppm, based on the total amount of the fluid. In anothernon-restrictive version herein, the amount of mineral oil may range froma lower end of about 50 independently to an upper end of about 8,000ppm. The amount of unsaturated fatty acid that may be effective in themethods and compositions may range from about 100 to about 20,000 ppm,based on the total amount of the fluid. In another non-restrictiveversion, the amount of unsaturated fatty acid may range from a lowerlimit of about 800 and/or to an upper limit of about 12,000 ppm.

Any suitable mixing apparatus may be used for the methods and fluidsherein. In the case of batch mixing, the VES and the aqueous fluid areblended for a period of time sufficient to form a gelled or viscosifiedsolution. The internal breaker, particularly the mineral oil, mayadvantageously be added after the fluid is formulated or at least afterthe fluid is substantially gelled. The VES that is useful herein may beany of the VES systems that are familiar to those in the well serviceindustry, and may include, but are not limited to, amines, amine salts,quaternary ammonium salts, amidoamine oxides, amine oxides, mixturesthereof and the like. Suitable amines, amine salts, quaternary ammoniumsalts, amidoamine oxides, and other surfactants are described in U.S.Pat. Nos. 5,964,295; 5,979,555; and 6,239,183, incorporated herein byreference in their entirety.

Viscoelastic surfactants improve the fracturing (frac) fluid performancethrough the use of a polymer-free system. These systems, compared topolymeric based fluids, can offer improved viscosity breaking, highersand transport capability, are in many cases more easily recovered aftertreatment than polymers, and are relatively non-damaging to thereservoir with appropriate contact with sufficient quantity of reservoirhydrocarbons, such as crude oil and condensate. The systems are alsomore easily mixed “on the fly” in field operations and do not requirenumerous co-additives in the fluid system, as do some prior systems.

The viscoelastic surfactants suitable for use herein include, but arenot necessarily limited to, non-ionic, cationic, amphoteric, andzwitterionic surfactants. Specific examples of zwitterionic/amphotericsurfactants include, but are not necessarily limited to, dihydroxylalkyl glycinate, alkyl ampho acetate or propionate, alkyl betaine, alkylamidopropyl betaine and alkylimino mono- or di-propionates derived fromcertain waxes, fats and oils. Quaternary amine surfactants are typicallycationic, and the betaines are typically zwitterionic. The thickeningagent may be used in conjunction with an inorganic water-soluble salt ororganic additive such as phthalic acid, salicylic acid or their salts.

Some non-ionic fluids are inherently less damaging to the producingformations than cationic fluid types, and are more efficacious per poundthan anionic gelling agents. Amine oxide viscoelastic surfactants havethe potential to offer more gelling power per pound, making it lessexpensive than other fluids of this type.

The amine oxide gelling agents RN⁺(R′)₂ O⁻ may have the followingstructure (I):

where R is an alkyl or alkylamido group averaging from about 8 to 24carbon atoms and R′ are independently alkyl groups averaging from about1 to 6 carbon atoms. In one non-limiting embodiment, R is an alkyl oralkylamido group averaging from about 8 to 16 carbon atoms and R′ areindependently alkyl groups averaging from about 2 to 3 carbon atoms. Inan alternate, non-restrictive embodiment, the amidoamine oxide gellingagent is Akzo Nobel's Aromox® APA-T formulation, which should beunderstood as a dipropylamine oxide since both R′ groups are propyl.

Materials sold under U.S. Pat. No. 5,964,295 include CLEARFRAC™, whichmay also comprise greater than 10% of a glycol. One preferred VES is anamine oxide. As noted, a particularly preferred amine oxide is APA-T,sold by Baker Oil Tools as SURFRAQ™ VES. SURFRAQ is a VES liquid productthat is 50% APA-T and greater than 40% propylene glycol. Theseviscoelastic surfactants are capable of gelling aqueous solutions toform a gelled base fluid. The internal breaker additives herein may beused to prepare a VES system sold by Baker Oil Tools as DIAMONDFRAQ™fracturing fluid system. DIAMONDFRAQ™ with its assured breakingtechnology overcomes reliance on external reservoir conditions in orderto break, as compared with products such as ClearFRAC™ fracturing fluidsystem.

The methods and compositions herein also cover commonly known materialsas AROMOX® APA-T manufactured by Akzo Nobel and other known viscoelasticsurfactant gelling agents common to stimulation treatment ofsubterranean formations.

The amount of VES included in the fracturing fluid depends on at leasttwo factors. One involves generating enough viscosity to control therate of fluid leak off into the pores of the fracture, and the secondinvolves creating a viscosity high enough to keep the proppant particlessuspended therein during the fluid injecting step, in the non-limitingcase of a fracturing fluid. Thus, depending on the application, the VESis added to the aqueous fluid in concentrations ranging from about 0.5to 25% by volume, alternatively up to about 12 vol % of the totalaqueous fluid (from about 5 to 120 gptg). In another non-limitingembodiment, the range for the present formulations is from about 1.0 toabout 6.0% by volume VES product. In an alternate, non-restrictive form,the amount of VES ranges from a lower limit of about 2 independently toan upper limit of about 10 volume %.

It is expected that the breaking compositions herein be used to reducethe viscosity of a VES-gelled aqueous fluid regardless of how theVES-gelled fluid is ultimately utilized. For instance, the viscositybreaking compositions could be used in all VES applications including,but not limited to, VES-gelled friction reducers, VES viscosifiers forloss circulation pills, fracturing fluids (including foamed fracturingfluids), gravel pack fluids, viscosifiers used as diverters in acidizing(including foam diverters), VES viscosifiers used to clean up drillingmud filter cake, remedial clean-up of fluids after a VES treatment(post-VES treatment) in regular or foamed fluid forms (i.e. the fluidsmay be “energized”) with or the gas phase of foam being N₂ or CO₂, andthe like.

A value of the methods and compositions herein is that a fracturing orother fluid can be designed to have enhanced breaking characteristics.That is, fluid breaking is no longer dependant on external reservoirconditions for viscosity break and is more controllable: the rate ofviscosity reduction, if complete break is achieved/occurs throughout thereservoir interval, and the like. Importantly, better clean-up of theVES fluid from the fracture and wellbore may be achieved thereby. Betterclean-up of the VES directly influences the success of the fracturetreatment, which is an enhancement of the well's hydrocarbonproductivity. VES fluid clean-up limitations and limitations of the pastmay be overcome or improved by the use of DiamondFRAQ™ improved VES gelclean-up technology and may now be improved by the use of breakerenhancers.

In order to practice the methods and compositions herein, an aqueousfracturing fluid, as a non-limiting example, is first prepared byblending a VES into an aqueous fluid. The aqueous fluid could be, forexample, water, brine, aqueous-based foams or water-alcohol mixtures.Any suitable mixing apparatus may be used for this procedure. In thecase of batch mixing, the VES and the aqueous fluid are blended for aperiod of time sufficient to form a gelled or viscosified solution. Asnoted, the breaking compositions herein may be added separately afterthe fluid is substantially gelled, in one non-limiting embodiment. Inanother non-restricted version, a portion or all of the breakingcomposition may be added prior to or simultaneously with the VES gellingagent if the breaking agent is in encapsulation form.

Propping agents are typically added to the base fluid after the additionof the VES. Propping agents may include, but are not limited to, forinstance, quartz sand grains, glass and ceramic beads, bauxite grains,walnut shell fragments, aluminum pellets, nylon pellets, and the like.The propping agents may be normally used in concentrations between about1 to 14 pounds per gallon (120-1700 kg/m³) of fracturing fluidcomposition, but higher or lower concentrations may be used as thefracture design required. The base fluid may also contain otherconventional additives common to the well service industry such as waterwetting surfactants, non-emulsifiers and the like. As noted herein, thebase fluid can also contain other non-conventional additives which cancontribute to the breaking action of the VES fluid, and which are addedfor that purpose in one non-restrictive embodiment.

Any or all of the above mineral oils, hydrogenated polyalphaolefin oils,saturated fatty acids, unsaturated fatty acids and/or breaker enhancersmay be provided in an extended release form such as encapsulation bypolymer or otherwise, pelletization with binder compounds, absorbed orsome other method of layering on a microscopic particle or poroussubstrate, and/or a combination thereof. Specifically, the internalbreakers and breaker enhancers may be micro- and/or macro-encapsulatedto permit slow or timed release thereof. In non-limiting examples, thecoating material may slowly dissolve or be removed by any conventionalmechanism, or the coating could have very small holes or perforationstherein for the internal breakers within to diffuse through slowly. Forinstance, a mixture of fish gelatin and gum acacia encapsulation coatingavailable from ISP Hallcrest, specifically CAPTIVATES® liquidencapsulation technology, may be used to encapsulate the internalbreakers and breaker enhancers herein. Also, polymer encapsulationcoatings such as used in fertilizer technology available from ScottsCompany, specifically POLY-S® product coating technology, or polymerencapsulation coating technology from Fritz Industries could possibly beadapted to the methods herein. The internal breakers may also beabsorbed onto zeolites, such as Zeolite A, Zeolite 13X, Zeolite DB-2(available from PQ Corporation, Valley Forge, Pa.) or Zeolites Na-SKS5,Na-SKS6, Na-SKS7, Na-SKS9, Na-SKS10, and Na-SKS13, (available fromHoechst Aktiengesellschaft, now an affiliate of Aventis S.A.), and otherporous solid substrates such as MICROSPONGE™ substrates (available fromAdvanced Polymer Systems, Redwood, Calif.) and cationic exchangematerials such as bentonite clay or placed within microscopic particlessuch as carbon nanotubes or buckminster fullerenes. Further, theinternal breakers may be both absorbed into and onto porous or othersubstrates and then encapsulated or coated, as described above.

In a typical fracturing operation, the fracturing fluid is pumped at arate sufficient to initiate and propagate a fracture in the formationand to place propping agents into the fracture. A typical fracturingtreatment would be conducted by mixing a 20.0 to 60.0 gallon/1000 galwater (volume/volume—the same values may be used with any SI volumeunit, e.g. 60.0 liters/1000 liters) amine oxide VES, such as SurFRAQ, ina 2 to 7% (w/v) (166 lb to 581 lb/1000 gal, 19.9 kg to 70.0 kg/m³) KClsolution at a pH ranging from about 6.0 to about 9.0. The breakingcomponents are typically added before or during the VES addition usingappropriate mixing and metering equipment, or if needed in a separatestep after the fracturing operation is complete or on the fly when goingdownhole. One unique aspect of the UFA breaking chemistry is how theplant, fish and like type oils may be added and dispersed within thebrine mix water prior to the addition of VES, such as the suction sideof common hydration units or blender tubs pumps. These oils, used at thetypical concentrations needed to achieve quick and complete break, donot initially act as detrimental oils and degrade VES yield and thelike. However, most other oils have a detrimental effect to VES yield ifalready present or when added afterwards. One novelty of the enoic-typeoils described herein is they are VES-friendly initially but over timeand a given temperature, or in the presence of the breaker enhancersbecome aggressive VES gel breakers. By “VES-friendly” is meant they arecompatible therewith and do not immediate decrease viscosity of aqueousfluids gelled with VES as is seen with many other oils.

In one non-limiting embodiment, the methods and compositions herein arepracticed in the absence of gel-forming polymers and/or gels or aqueousfluids having their viscosities enhanced by polymers. However,combination use with polymers and polymer breakers may also be ofutility. For instance, polymers may also be added to the VES fluidsherein for fluid loss control purposes. Types of polymers that may serveas fluid loss control agents are various starches, polyvinyl acetates,polylactic acid, guar and other polysaccharides, gelatins, and the like.

The present invention will be explained in further detail in thefollowing non-limiting Examples that are only designed to additionallyillustrate the invention but not narrow the scope thereof.

GENERAL PROCEDURE FOR EXAMPLES

To a blender were added tap water, the indicated proportion of KCl,followed by the indicated proportion of viscoelastic surfactant(WG-3L-AROMOX® APA-T VES available from Akzo Nobel). The blender wasused to mix the components on a very slow speed, to prevent foaming, forabout 30 minutes to viscosity the VES fluid. Mixed samples were thenplaced into plastic bottles. Various components singly or together, invarious concentrations, were then added to each sample, and the samplewas shaken vigorously for 60 seconds. The samples were placed in a waterbath at the indicated temperature and visually observed every 30 minutesfor viscosity reduction difference between the samples. Since a goal ofthe research was to find a relatively rapid gel breaking composition,samples were only observed for 24 hours or less.

Viscosity reduction may be visually detected. Shaking the samples andcomparing the elasticity of gel and rate of air bubbles rising out ofthe fluid may be used to estimate the amount of viscosity reductionobserved. Measurements using a Brookfield PVS rheometer at the indicatedtemperatures and pressures at 100 sec⁻¹ were used to acquirequantitative viscosity reduction of each sample.

Formulations 1-4 of FIG. 1

The four formulations which gave the results shown in FIG. 1 had thefollowing compositions:

-   Formulation #1: Adriatic seawater+4% KCl+2.5% WG-3L VES+4.0 gptg    GBW-406L internal breaker (refined soybean oil (polyenoic acid) from    Welch, Holmes & Clark Company)-   Formulation #2: Adriatic seawater+4% KCl+2.5% WG-3L VES+3.0 gptg    GBW-406L internal breaker+0.4 gptg GBC-4L transition metal ion rate    controller “catalyst” (15% bw CuCl₂. 2H₂O solution)-   Formulation #3: Adriatic seawater+4% KCl+2.5% WG-3L VES+3.0 gptg    GBW-406L internal breaker+2.0 gptg GBC-6L breaker enhancer+0.05 gptg    GBC-4L transition metal ion rate controller “catalyst”. GBC-6L    breaker enhancer is 30% by volume (bv) SPAN® 80 sorbitan monooleate    +70% bv ConocoPhillips Pure Performance Base (mineral) Oil 225N.-   Formulation #4: Adriatic seawater+4% KCl+2.5% WG-3L VES+3.0 gptg    GBW-406L internal breaker+2.0 gptg GBC-6L breaker enhancer+0.4 gptg    GBC-4L transition metal ion rate controller “catalyst”.

FIG. 1 presents curves for each Formulation showing the viscositybreaking results as a function of time. Formulation 1 which simply hadthe polyenoic acid breaker (soybean oil) had the most stable viscosity.The addition of the copper chloride solution rate controller which gaveFormulation 2started breaking the viscosity around 16 hours. With theaddition of the sorbitan monooleate breaker enhancer herein and mineraloil internal breaker in Formulation 3 (having less of the ratecontroller), the viscosity broke quickly starting at about 4 hours,where Formulation 4 also having the sorbitan monooleate breaker enhancerherein and mineral oil internal breaker and the same amount of ratecontroller as in Formulation 2 broke the fastest at about 1 hour. Theseformulations showed that the breaker enhancers effectively acceleratedbreaking in high salinity and unique salinity composition mix water.

Formulations of FIG. 2

The aqueous base fluid used in the experiments of FIG. 2 had 9% KCl, 6%WG-3L VES, 5.0 gptg FLC-40L fluid loss control agent (slurried MgOpowder mixed in monopropylene glycol available from Baker Oil Tools).The FLC-40L agent was added to the mix water before the WG-3L VES. Nobreakers were added to the formulation that is the most stable in FIG.2. In the second formulation the FLC-40L agent, 6 gptg GBW-407L internalbreaker, 6.0 gptg GBW-402L internal breaker and 0.3 gptg GBC-4L ratecontroller were added to the mix water before the WG-3L VES. Thisformulation gave a noticeable break beginning at about 2 hours. TheGBW-407L internal breaker was Fish Oil 18:12 TG high EPA and DHAunsaturated fatty acid oil product from Bioriginal Food & ScienceCorporation. GBW-402L internal breaker was ConocoPhillips PurePerformance Base Oil 225N type mineral oil—a fairly high molecularweight mineral oil. The third formulation was prepared similarly exceptthat 3.0 gptg GBW-402L internal breaker was used along with 1.0 gptgGBC-6L breaker enhancer. This formulation broke the fastest beginning atless than 1 hour further demonstrating that the breaking enhancers ofthe present methods and compositions are demonstrably effective. It maybe noticed how less GBW-402L mineral oil internal breaker was required,but that still faster VES breaking occurs when GBC-6L breaker enhanceris present.

Formulations of FIG. 3

The aqueous base fluid used in the experiments of FIG. 3 had 3% KCl and4% WG-3L VES. The formulation with no breakers was the most stable withno viscosity decrease in the 12 hours of FIG. 3. A second formulation ofthe aqueous base fluid with 4 gptg GBW-406L internal breaker and 0.3gptg GBC-4L transition metal ion rate controller added to the mix waterbefore the WG-3L VES gave the first breaking curve of FIG. 3. The thirdformulation was similar to the second formulation except that it alsoincluded 0.25 gptg GBC-6L breaker enhancer which from the next lowestcurve of FIG. 3 shows definite enhancement of breaking the viscosity. Inthe fourth formulation, the GBC-6L breaker enhancer was replaced by 2.0gptg Exp-LS90 breaker enhancer (lecithin product EDS90 (phospholipids)from ADM Corporation) which gave a slightly faster break than the thirdformulation. The fastest viscosity break was given by the fifthformulation which was identical to the third and fourth formulationsexcept that the breaker enhancer was 2.0 gptg GBC-7L breaker enhancer.This breaker enhancer was 20 by volume (bv) SPAN® 85 sorbitantrioleate+80 bv ConocoPhillips Pure Performance Base (mineral) Oil 225Ninternal breaker. For this fifth formulation the initial upper baselineviscosity with no breakers was not reached.

As can be seen, the method of gel breaking described herein is simple,effective, safe, and highly cost-effective. A method is provided forenhancing the breaking the viscosity of aqueous treatment fluids gelledwith viscoelastic surfactants (VESs). Compositions and methods are alsofurnished herein for breaking VES-surfactant fluids controllably,completely and relatively quickly with the breaker enhancers describedherein.

Compositions and methods are also disclosed herein for enhancing thebreaking of VES-surfactant fluids where contact with reservoir fluids'external breaking mechanism is not required, although in someembodiments heat from the reservoir may help the breaking process.Compositions and methods are additionally provided for breakingVES-surfactant fluids where the breaking additive and breaker enhancerare in a phase internal to the VES-surfactant fluid. Further, methodsand VES fluid compositions are described herein for breaking theviscosity of aqueous fluids gelled with viscoelastic surfactants usingreadily available materials at relatively inexpensive concentrations.

In one non-limiting explanation, it appears the oil-soluble surfactantsbecome dispersed throughout the VES micelles and appear to then bethermodynamically weak points where polyenoic autooxidation productsrequire less thermodynamic energy to disturb the outer layer of“associated hydrophillic head groups and counterions” (K, Ca, etc.counterions) to degrade VES micelle viscosity by micelle rearrangement.This method to degrade VES micelles is not seen as spontaneous or veryabrupt but rather a gradual over time mechanism for a given fluidtemperature, VES loading, mix water salinity, type and amount ofoil-soluble surfactant, and the like, that can be crafted to allowcontrol viscosity break over time, and can give an enhanced viscositybreaking compared to if polyenoic and/or mineral oil breakers alone areused to access, disassociate, and/or disturb the VES micelles and reducethe fluid viscosity.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof, and has been demonstrated aseffective in providing methods and compositions for a VES fracturingfluid breaker mechanism. However, it will be evident that variousmodifications and changes can be made thereto without departing from thebroader spirit or scope of the invention as set forth in the appendedclaims. Accordingly, the specification is to be regarded in anillustrative rather than a restrictive sense. For example, specificcombinations of viscoelastic surfactants, internal breakers, breakerenhancers, and other components falling within the claimed parameters,but not specifically identified or tried in a particular composition orfluid, are anticipated to be within the scope of this invention.

The word “comprising” as used throughout the claims is to be interpretedto mean “including but not limited to.”

1. A method for breaking the viscosity of aqueous fluids gelled with aviscoelastic surfactant (VES) comprising adding to an aqueous fluid atleast one VES in an amount effective to increase the viscosity thereof;adding to the aqueous fluid before or after the VES at least oneinternal breaker in an amount effective to reduce the viscosity of thegelled aqueous fluid, where the internal breaker is selected from thegroup consisting of polyenoic acids and monoenoic acids; and adding tothe fluid before, during or after adding the internal breaker, at leastone oil-soluble surfactant breaker enhancer in an amount effective toaccelerate reduction of viscosity of the gelled aqueous fluid.
 2. Themethod of claim 1, where the oil-soluble surfactant breaker enhancer isselected from the group consisting of sorbitan fatty acid esters,saponified hard oils, saponified hydrogenated fatty acid oils, longchain alcohols, long chain fatty acid alcohols, long chain fatty amines,long chain sulfates, long chain sulfonates, phospholipids, and lignins.3. The method of claim 1 where the internal breaker is present in anamount from about 100 to about 20,000 ppm based on the total fluid; andthe breaker enhancer is present in an amount from about 0.001 to about10 wt % based on the total fluid.
 4. The method of claim 1 where theinternal breaker is a polyenoic acid selected from the group consistingof linoleic acid, omega-3 fatty acids, omega-6 fatty acids, stearidonicacid, eleostearic acid, eicosadienoic acid, eicosatrienoic acid,arachidonic acid or eicosatetraenoic acid, eicosapentaenoic acid,docosapentaenoic acid, docosahexaenoic acid, cis-linoleic acid,cis-linolenic acid, gamma-linolenic acid, and conjugated polyenes. 5.The method of claim 1 where the internal breaker is a monoenoic acidsselected from the group consisting of obtusilic acid, caproleic acid,lauroleic acid, linderic acid, myristoleic acid, physeteric acid,tsuzuic acid, palmitoleic acid, petroselinic acid, oleic acid, vaccenicacid, gadoleic acid, gondoic acid, cetoleic acid, nervonic acid, erucicacid, elaidic acid, and t-vaccenic acid.
 6. The method of claim 1 wherethe internal breaker is present in an oil-soluble internal phase of theaqueous fluid.
 7. The method of claim 1 where the viscosity is brokenwithin about 1 to about 16 hours at a temperature from about 25° C. toabout 150° C.
 8. A method for breaking the viscosity of aqueous fluidsgelled with a viscoelastic surfactant (VES) comprising adding to anaqueous fluid at least one VES in an amount effective to increase theviscosity thereof; adding to the aqueous fluid before or after the VESat least one internal breaker in an amount effective to reduce theviscosity of the gelled aqueous fluid, where the internal breaker isselected from the group consisting of polyenoic acids and monoenoicacids; and adding to the fluid before, during, or after adding theinternal breaker, from about 0.001 to about 10 wt % based on the totalfluid of at least one oil-soluble surfactant breaker enhancer, where theoil-soluble surfactant breaker enhancer is selected from the groupconsisting of sorbitan fatty acid esters, saponified hard oil,saponified hydrogenated fatty acid oils, long chain alcohols, long chainfatty acid alcohols, long chain fatty amines, long chain sulfates, longchain sulfonates, phospholipids, and lignins.
 9. The method of claim 8where the internal breaker is present in an amount from about 100 toabout 20,000 ppm based on the total fluid.
 10. The method of claim 8where the internal breaker is present in an oil-soluble internal phaseof the aqueous fluid.
 11. The method of claim 8 where the viscosity isbroken within about 1 to about 16 hours at a temperature from about 25°C. to about 150° C.